In known processes of electrostatic imaging, a light image of an original to be copied may be recorded in the form of an electrostatic latent image upon a photoconductive member. A developed image may then be obtained from the latent image by application of toner particles, which may be in the form of a finely powdered solid or in liquid phase.
In `dry` electrostatic imaging processes which employ powdered solid toners, it is known to pass a substrate having a developed image formed thereon between (for example) a pair of opposed rollers. One or both of the rollers is heated and in consequence heats and thereby fuses and fixes the image on the substrate, insofar as at least to some extent the image particles flow into the fibers or pores of the substrate. For prior patents in the field of fusing and fixing the developed image, reference is made to U.S. Pat. Nos. 3,249,738, 3,637,976, 3,667,742, 3,718,116 and 4,689,471 the disclosures of which are incorporated herein by reference.
In dry electrostatic imaging processes as described above, unintended offsetting of the developed image onto one of the rollers may occur. It is known that the incidence of offsetting may be reduced by employing a roller covered with polytetrafluoroethylene or silicone rubber, to which a release agent such as silicone oil is applied, which release agent apparently forms an interface between the roller surface and the toner images on the support.
As has been indicated above, liquid toner based electrostatic imaging is known. The basic steps of the liquid toner process are:
1. Generating an electrostatic latent image, e.g. on a photoconductive surface such as a plate or drum which has been sensitized by charging with a corona discharge and by exposure to light in the form of an image of an object--the charge is dissipated in exposed areas;
2. Developing the latent image by contact with a liquid dispersion ("toner") of fine particles which in a positive process are attracted to the charged areas and which in a reverse process are attracted to the uncharged areas;
3. Removal of excess liquid toner and particles from the background areas;
4. Transfer of the image, e.g. to a substrate such as paper, under influence of an electrical field;
5. Fixing the image by fusing the particles together and to the substrate; and
6. Cleaning the plate or drum for reuse.
In this connection reference is made to the following published patent applications and issued patents in the field of electrophotography: GB Published Patent Applications Nos. 2,169,416A and 2,176,904A and U.S. Pat. Nos. 3,990,696, 4,233,381, 4,253,656, 4,256,820, 4,269,5o4, 4,278,884, 4,286,039, 4,302,093, 4,326,644, 4,326,792, 4,334,762, 4,350,333, 4,355,883, 4,362,297, 4,364,46o, 4,364,657, 4,364,661, 4,368,881, 4,378,422, 4,392,742, 4,396,187, 4,4oo,079, 4,411,976, 4,412,383, 4,413,048, 4,418,903, 4,420,244, 4,435,o68, 4,439,035, 4,454,215, 4,46o.667, 4,473,865, 4,480,825, 4,501,486, 4,522,484, 4,531,824, 4,538,899, 4,582,774, 4,585,329, 4,586,81o, 4,589,761, 4,598,992, 4,603,766, 4,620,699, 4,627,705 and 4,678,317, the disclosures of all of which are incorporated by reference herein.
Following development, the excess liquid toner and particles are removed from the image, by a step often referred to as "metering". This step may be carried out by use of an electrically biased reverse roller which also shears off the "fluffy" part of the imaged regions and squeezes or compacts the imaged area to prepare it for transfer. At the transfer stage, there is likely to occur a degree of smudging, smearing or squashing of the image, which detracts from its quality. It will be appreciated that if in the fusing and fixing stage, heat and pressure are applied to the liquid toner image, as has been described above for solid toner based processes, the liquid image will be likely to again suffer from smudging, smearing or squashing.
In Published UK Patent Application GB 2169416A (Landa et al), the disclosure of which is incorporated herein by reference, a degree of squash-resistance is imparted to the developed image by using fibrous toner particles prepared as follows: a thermoplastic polymer (and pigment) is plasticized with a nonpolar liquid, preferably at elevated temperature, to form a sponge, the sponge is shredded, more nonpolar liquid is added, the pieces of shredded sponge are wet-ground into particles and the grinding is continued to pull the particles apart to form fibers extending therefrom (the particles preferably having a diameter less than 5 microns). Finally, a charge director may be added to impart a charge of predetermined polarity to the toner particles.
While the method of the aforementioned Published UK Patent Application includes the imparting of squash resistance to the image, it would nevertheless be desirable to fix a liquid image on a substrate by analogy with the application of heat and pressure in dry electrostatic imaging processes, but, as has been indicated above, this has not been possible hitherto because of the liability of liquid toner based images particularly, to smudging, smearing or squashing.